1. Field of the Invention
This invention relates to a developing device for use in electrophotographic machines such as electrophotographic copying machines, electrostatic printing machines, facsimile machines and the like for developing an electrostatic latent image formed on an imaging surface such as a photosensitive member, and, in particular, it relates to a developing device including a developing electrode whose potential is controlled to prevent toner particles from being excessively adhered to the imaging surface.
2. Description of the Prior Art
In wet-type electrophotographic image processing technology, an electrostatic latent image is first formed on an imaging surface and the thus formed latent image is developed by a liquid developer, which is comprised of carrier liquid and toner particles having charges opposite in polarity to the charges forming the latent image and dispersed in the carrier liquid, followed by the step of transferring the thus developed image onto a transfer medium such as paper by means of electrostatic or mechanical pressure means. Such a wet-type developing device usually includes a developing electrode, generally in the form of a curved plate, which is disposed at the bottom of a photosensitive drum with a predetermined gap therebetween. And, a liquid developer is supplied to fill this gap whereby an electrostatic latent image formed on the peripheral surface of the drum is developed. In this case, toner particles charged to a predetermined polarity and dispersed in the carrier liquid are selectively attracted to the imaging or peripheral surface of the drum in accordance with the electric field formed between the developing electrode and that portion of the imaging or peripheral surface of the drum opposite to the developing electrode. When the potential of the developing electrode is set such that it is slightly higher than the potential of the background portion and lower than the potential of the image portion on the imaging surface, it may be so controlled that toner particles adhere only to the image portion and not to the background portion thereby allowing to prevent the occurrence of so-called background contamination.
One prior art approach for controlling the potential of a developing electrode is to apply a fixed bias potential to the developing electrode. Another prior art approach is to provide an electrically floating developing electrode, the potential of which floats as induced by the potential of the developing electrode between predetermined upper and lower limits so as to prevent the deposition of toner particles to the background portion, as disclosed in Japanese Patent Laid-open Publication, No. 51-48341. However, in the former case, problems will be encountered if the potential of the background portion itself increases as in the case where the background portion of an original is colored. The latter case is also not free from disadvantages; for example, if an original image is not a line image and it has a continuously varying gray scale as in the case of a photograph, the potential of the developing electrode will be increased excessively thereby causing excessive deposition of toner particles. This is particularly disadvantageous for modern electrophotographic copying machines designed to supply an increased amount of toner so as to meet the requirements for obtaining high density images and for carrying out the so-called paper-free process, i.e., the process which can be carried out without preference to the kind of paper to be used as a transfer medium.